The present invention relates to an apparatus for the surface treatment of galvanized sheet-iron or plates.
It is known that galvanized sheet-iron has in general a surface appearance usually referred to as a "large flower" configuration or pattern on the solidified zinc layer. In certain cases the users may argue against this appearance and request the delivery of sheet-iron stock having the so-called "minispangle" pattern, that is, surface designs having a size of the order of one millimeter, imparting a semi-bright appearance to the plate. This specific configuration is attended by a considerable uniformity in the thickness of the zinc layer.
This very small flower pattern may be obtained notably by spraying finely divided solid particules onto the zinc layer covering the sheet, before the liquid zinc has crystallized completely. Such finely divided solid particles must compulsorily consist of a material capable of undergoing a thermal transformation while in contact with the liquid zinc layer, this transformation process comprising at least one melting step and a decomposition step.
As a rule, particles consisting of pulverulent zinc capable of producing a great number of crystalline seeds when in contact with liquid zinc constituting the sheet-iron layer, during the solidification thereof, are used. The crystals thus created by these seeds are not only extremely numerous but also capable of assuming extremely small dimensions, so that very minute "flowers" are formed on the solidified zinc surface coating. This process is usually referred to as "minispangle".
In known "minispangle" methods the application of pulverulent particles, is carried out by spraying particles preferably of zinc, to the still liquid zinc layer, the particles being propelled at a speed high enough to enable them to pass through the ascending stream of hot air developing in close vicinity of the sheet surface as the sheet emerges from the galvanization bath, and to contact intimately the melting zinc layer deposited on the sheet surface. For this purpose, carrier-gas spraying means such as air for projecting at high speed a suspension of fine zinc particles onto the galvanized strip as it emerges from the zinc melt is used.
As a consequence of this blowing action, zinc is caused to set on the strip surface and the size of the "flowers" thus formed is subordinate to the amount of zinc particles per cubic unit of blown air.
In known apparatus for carrying out the above-mentioned method air is blown perpendicularly to the sheet by means of a pair of nozzles disposed on either side of the sheet fed continuously from the galvanizing bath.
As a rule, after flowing along the sheet this air is recovered and sent to the blower and; escaping air is sent to a filter.